1. Field of Invention
This invention relates to integrated circuit (IC) fabrication, and particularly to an overlay mark for checking the alignment accuracy between a former layer and a later layer on a wafer, and a method of checking alignment accuracy with the overlay mark.
2. Description of Related Art
Overlay marks are generally formed on a wafer to check the alignment accuracy between a former layer and a later layer. Currently, the most popular type of overlay mark is the so-called box-in-box (BIB) overlay mark.
FIG. 1 shows a top view of a prior art BIB overlay mark together with patterns in a device region. The BIB overlay mark typically includes, in an overlay mark region 14 usually in the scribe line region (not shown) of a semiconductor wafer, a square-ring pattern 100b as a part of a former layer, and a box pattern 110b as a part of the patterned photoresist layer defining a later layer formed after the former layer is patterned. The pattern 100b is defined together with the pattern 100a of the former layer in the device region 12. The pattern 110b is defined together with the pattern 110a of the later layer in the device region 12, and is surrounded by the ring pattern 100b. It is expected that the x- and y-shifts of the box pattern 110b relative to the ring pattern 100b reflect the x- and y-directional overlay errors between the former layer 100a and the later layer 110a in the device region 12.
However, when the device patterns are not oriented in the x- or y-direction of the wafer, the conventional overlay mark does not have the same response as is the patterns in the device region and hence can no longer reflect the behaviors of the device features. A reason causing this problem is that the rotated patterns in combination with the illumination source induce various aberrations to which the device patterns and the above overlay mark have different responses. For example, as illustrated in FIG. 1, the box pattern 110b in the overlay mark region 14 may be shifted to the opposite direction in the x-axis direction in relation to the shifted pattern 110a of the later layer in the device region 12. The positional shift of the box pattern 110b in the x- or y-direction may be different from that of the pattern 110a in the device region 12 due to aberrations.